Time-of-flight mass spectrometry is a widely used tool of analytical chemistry, characterized by a high speed of analysis in a wide mass range. Multi-reflecting time-of-flight mass spectrometers (MR-TOF MS) enable substantial increases in resolving power due to the flight path extension. Such flight path extension requires the folding of ion path trajectories. Reflecting the ions in mirrors is one method for accomplishing the folding of ion paths. UK Patent No. GB2080021, by inventor H. Wollnikas, appears to have disclosed the potential for utilizing mirrors to reflect ions. The deflection of ions in sector fields provides a second method for accomplishing the folding of ion paths. This second method appears to have been disclosed in a 2003 scholarly article attributed to Japan's Osaka University. See Michisato Toyoda et al., Multi-Turn Time-of-Flight Mass Spectrometers with Electrostatic Sectors, 38 J. Mass Spectrometry 38 1125 (2003). Of these two methods for folding ion paths, mirror-type MR-TOF MS, due to their high-order time per energy focusing, allow for larger energy acceptance, which is an important advantage.
As far back as 1989, an advanced scheme of folded-path MR-TOF MS using two-dimensional (planar) gridless mirrors was known. The Russian Patent No. SU 1725289, by Nazarenko et. al., appears to have utilized this scheme, which is illustrated in the present FIG. 1. The planar mass spectrometer by Nazarenko provides no ion focusing in the z-direction; thus, essentially limiting the number of reflection cycles.
The present inventors, in Publication No. WO2005001878, appear to have disclosed a set of periodic lenses in the field-free region between the planar ion mirrors to confine ion packets in the drift z-direction. The present FIG. 2 illustrates a MR-TOF MS utilizing these periodic lenses.
The present inventors, in UK Publication No. GB2476964, appear to have disclosed curved ion mirrors in the drift z-direction forming a hollow cylindrical electrostatic ion trap, further extending the ion flight path within a MR-TOF MS.
Increasing the flight path length in the MR-TOF MS causes three distortions (aberrations) to the flight time (TOF), each of which limit the mass resolving power. The three aberrations are: (i) ion energy spread, (ii) spatial spread of ion packets in the y-direction, and (iii) spatial spread of ion packets in the z-direction. The z-directional spatial spread aberrations are primarily the second order TOF aberrations (“T|zz”) referred as the “spherical” aberration. A spherical aberration is created by periodic lenses confining the ion beam in the z-direction and is always positive (T|zz>0).
The present inventors, in Publication No. WO2013063587, appear to disclose an improvement to the ion mirror isochronicity with respect to energy and y-spread. Thus, T|zz aberrations caused by the periodic lenses remain the major remaining TOF aberration limiting the mass resolving power of the MR-TOF MS.
To reduce those T|zz aberrations, the present inventors, in U.S. Patent Application No. 2011186729, appear to disclose a quasi-planar ion mirror comprising, in essence, a spatially and periodically modulated ion mirror field as illustrated in FIG. 3. The spatially modulated ion mirror field provides for negative T|zz aberration, thus compensating for the positive T|zz caused by the periodic lenses utilized in MR-TOF MS.
Even so, numerical simulations of MR-TOF MS with quasi-planar ion mirrors show that such mirrors achieve efficient elimination of TOF aberrations only if the period of the electrostatic field inhomogeneity in the z-direction equals or exceeds the y-height of the mirror window. Hence, in the field of MR-TOF MS, practical analyzer sizes continue to limit the density of ion trajectory folding and the flight path extension. What is more, the fact that periodic modulation affects y-components of the field and complicates the analyzer tuning presents another limitation.
Accordingly, a need exists in the art to provide an alternative way of reducing the spherical TOF aberrations T|zz, which can be used in planar or hollow cylindrical MR-TOF MS with densely folded ion trajectories and can provide for technical simplicity and decoupling of tuning of ion-optical properties in y- and z-directions.